1. Field of the Invention
This invention relates to a gas sensor for detecting a specific-component concentration in a gas. In addition, this invention relates to a method of detecting a specific-component concentration in a gas.
2. Description of the Related Art
European patent application publication number 0678740 A1 corresponding to Japanese patent application publication number 8-271476 discloses a sensing device for measuring a concentration of a gas component of a measurement gas. The sensing device in European application 0678740 A1 includes a first internal space into which the measurement gas is introduced from an external measurement-gas space via a first diffusion controlling passage. A flow of the measurement gas toward the first internal space receives a predetermined diffusion resistance provided by the first diffusion controlling passage. The first internal space communicates with a second internal space via a second diffusion controlling passage. The atmosphere is introduced from the first internal space into the second internal space via the second diffusion controlling passage under a predetermined diffusion resistance provided by the second diffusion controlling passage. A first oxygen pumping means operates to control an oxygen partial pressure in the first internal space. A second oxygen pumping means operates to pump out oxygen from the atmosphere in the second internal space toward a reference-gas channel. An ammeter detects a pumping current caused by the pumping action of the second oxygen pumping means. The concentration of the gas component of the measurement gas is measured from the detected pumping current.
U.S. Pat. No. 5,985,118 corresponding to Japanese patent application publication number 10-185868 discloses a gas concentration detector using solid electrolyte layers laminated. The gas concentration detector measures a concentration of gas constituents in measuring gas such as exhaust gas of an internal combustion engine without being influenced by oxygen concentration in the measuring gas. The measuring gas is introduced into an inner cavity of the detector, and outside air as a reference gas is introduced into an air passage in the detector. Oxygen concentration in the measuring gas in the inner cavity is maintained at a predetermined level by operation of an oxygen pumping cell constituted by an ion conductive solid electrolyte layer and a pair of electrodes formed on both surfaces of the layer. The oxygen concentration in the inner cavity is measured by an oxygen sensor cell having one electrode exposed to the measuring gas in the inner cavity and the other electrode exposed to the reference gas. The concentration of the gas constituents in the measuring gas is measured in terms of ion current generated in a detector cell constituted by an ion conductive solid electrolyte layer and a pair of electrodes formed on both surfaces of the layer, one electrode being exposed to the measuring gas in the inner cavity and the other exposed to the reference gas in the air passage. Either one of the sensor cell and detector cell electrodes exposed to the measuring gas in the inner cavity is made of a material active to the gas constituents to be measured and the other electrode is made of a material inactive thereto.
As will be explained later, a prior-art hydrocarbon gas sensor tends to have an error in the result of detection of a hydrocarbon gas concentration in the case where measurement environments vary.